In general, in order to reduce access time of an optical disk apparatus, it is important to reduce the size and weight of optical heads. In conventional systems, a track signal, a focus signal and a rf (high frequency) signal are detected by individual optical systems which require a plurality of optical parts with a corresponding complexity of structure. The complicated structure both increases the weight of the optical head and the access time. Attempts have been made at reducing the size and weight of optical pick-ups by utilizing high-density gratings; however, with high-density gratings, the angle of diffracted light tends to deviate due to fluctuations in the wavelength of laser light emanating from the semiconductor laser. In addition, it is necessary to space two photosensor elements, one for detecting a focusing signal and the other for detecting a track signal, apart from one another by a large distance, which leads to difficulty in assembly and adjustment, and which increases the size of the device.
An approach to this problem is to utilize an optical head which employs a dual diffraction grating for separating light reflected from the optical disk to perform the light signal detecting function. Exemplary of this approach is the disclosure of U.S. Pat. No. 5,029,154 to Sumi et al., in which a quartz substrate has diffraction gratings etched on opposite sides thereof. Sumi et al. is in contrast to approaches such as that of U.S. Pat. No. 4,945,525, in which two separate diffraction gratings in spaced relation to one another are used to separate a beam of coherent light into first and second components, such as TE and TM components, for use in generating tracking error, focusing error and information signals.
While U.S. Pat. No. 5,029,154 discloses the concept of a dual-diffraction grating, the light beams are separated by utilizing gratings of different pitch or different geometrical configuration. Moreover, in Sumi et al., the gratings are on opposite sides of a single substrate. Once one side of a substrate has been etched to provide a first grating, it is extremely difficult to protect the first grating while fabricating the second grating. Moreover, with the approach of Sumi et al., it is necessary to insert a focusing lens between the double grating and the beam reflected from the optical disk, which introduces astigmatism in the optical beam which results in cross-talk between the focusing and tracking signals.
In view of the Sumi et al. structure, and the difficulties associated therewith, there is a need for improvement in and alternative approaches to the dual diffraction grating concept.